1. Field of the Invention
The present invention relates generally to electroless silver plating processes and systems based upon the autocatalytic plating of silver onto various substrates. More particularly, the present invention relates to a process involving the control of the amount of oxygen in the electroless silver plating solution to provide a stabilized solution which is regenerated to the active plating solution by partially removing the oxygen with a scrubber gas, immediately prior to the plating process. The rate of autocatalytic silver deposition and the physical characteristics of the resulting silver plating is controlled in part by the oxygen content of the active electroless silver plating solution.
2. Description of the Related Art
Electroless plating is based upon the autocatalytic or spontaneous decomposition of a metal compound in a plating solution to provide deposition and plating of the metal onto a particular substrate which is immersed in the solution. Electroless nickel plating processes and electroless copper plating processes are well known and commercially widely used. Processes for electroless plating of gold, platinum, palladium and silver are also well known.
Electroless silver plating has particular application to the plating of passive microwave components. Passive microwave components are fabricated by machining, casting, dip brazing, electroforming or resin composite methods. The materials used are aluminum, invar (a nickel-iron alloy), and other metal and resin composites. Generally these components are silver electroplated to improve the Quality Factor (Q Factor) of the device, which is a measure of how well the device performs as a microwave cavity. The electrical performance or Q Factor of passive microwave components is directly related to the geometry of the component, surface smoothness, and the specific conductance of its energy propagating surfaces. To provide the desired electrical performance, the surfaces are typically silver electroplated to a thickness of between 100 to 1000 microinch (2.54 to 25.4 micrometers) depending on the energy frequency design of the component. Unfortunately, conventional electroplating does not deposit silver uniformly onto all the component surfaces. External surfaces rapidly build up a silver layer while internal, recessed surfaces are barely covered. The result can be a detrimental change of critical dimensions in high current density areas and insufficient silver deposit or plating at recessed surfaces. Although the insufficient and irregular silver thickness resulting from electroplating causes the Q Factor to be less than desired, the silver deposit conductance is still quite good and in spite of these difficulties generally provides a better overall Q Factor for the component than if no silver plating is provided.
The problems with non-uniform silver deposit thickness on complex structures resulting from electroplating can be overcome if the deposit is made by electroless or autocatalytic silver deposition. Electroless or autocatalytic silver plating is capable of depositing a silver layer uniformly over any geometry and is especially well suited for plating passive microwave components.
Although electroless silver technology is well established, silver electroless plating has not become a commercial technology because the plating baths tend to spontaneously decompose forming silver particles throughout the solution. This decomposition causes loosely adherent, very fine silver metal particles to be deposited roughly on the plating surface at some unknown time during plating. The result is an unacceptable microwave silver deposit. This plating bath instability tendency can be slightly, but not sufficiently, decreased by adjustment of the solution concentrations. However, these adjustments lower the silver deposition rate to 0-3 micrometers (.mu.m)/hour compared to about 10 .mu.m/hour for commercial electroless nickel.
One electroless silver solution which finds occasional use is described by F. Pearlstein and R. F. Weightman, "Electroless Deposition of Silver Using Dimethylamine Borane", Plating, February 1974, pp. 154-157 and is an aqueous solution containing sodium silver cyanide [NaAg(CN).sub.2 ], free sodium cyanide (NaCN), sodium hydroxide (NaOH), and dimethylamine borane (DMAB) in the following proportions:
1.83 g/l NaAg(CN).sub.2 PA1 1.0 g/l NaCN PA1 1.0 g/l NaOH PA1 2.0 g/l DMAB
This solution above is formulated for maximum plating speed. Thiourea at about 0.5 mg/liter is usually added to this bath to improve stability, but has the detrimental effect of altering the properties of the deposit.
Thus, it would be desirable to provide a process for the electroless plating of silver in which the plating bath or solution is stabilized while adequate deposition rates and plating characteristics are maintained to provide desired uniform metal deposition on passive microwave components and other substrates having complex structures where uniform plating thickness and quality is required.
Various different plating surface textures are desirable depending upon the particular application for which the plated surface is to be used. For example, a smooth silver deposit is usually desirable in certain critical applications such as microwave waveguide components to decrease signal losses, or optical devices and bearings to improve performance. In other situations, it is desirable to provide a plating surface which is rough or dendritic to allow adhesion of various plastics or other materials to the plated surface.
It would be desirable to provide an electroless silver plating process in which the physical characteristics of the deposited plating can be varied accurately and conveniently to provide desired surface textures.